The invention relates to a method for holding together a blow mold of multipiece design of a blowing station. Furthermore, the invention relates to a blowing station for a blowing machine, and also to a blowing machine.
Blowing machines for the blow molding of containers consisting of a thermoplastic material have been known for quite some time in the prior art. The containers are molded by the action of blowing compressed air in so-called blow molds. The cavity inside a blow mold forms the female mold for the container which is to be produced. In blowing machines, which work according to the principle of rotary machines, the blow molds are arranged at blowing stations which are located in the region of blowing wheels. In order to blow a container inside a blow mold, the blow mold is first of all opened and a so-called preform inserted in the blow mold. Blow molds usually consist of a plurality of segments, especially three segments, specifically typically consisting of a mold bottom and two side halves with a semi-cylindrical base contour. These blow mold segments are as a rule detachably fastened on stable blow mold carriers. The detachability enables an exchange of molds if a change is to be made from one bottle type to another bottle type which is to be produced.
For opening and closing the blow mold, the blow mold segments are moved. Typically, it involves the capability of opening up the side sections in the manner of a book and a lifting and lowering of the mold bottom. Other movements are also possible. The basic construction of a blowing station with blow mold carriers and a multipiece blow mold is described in EP 1 919 690 B1, for example.
On account of the high pressures which prevail inside the blow mold during a blow molding process, there is a necessity for locking mechanisms for the closing and clamping of the blow mold, which ensure a lock which is tight and secured against inadvertent opening of the blow mold. Such locking mechanisms are known under the term mold clamp, for example.
It is known to pressurize a mold clamp for the clamping of the blow mold with a blowing pressure, which is provided for the blowing of the containers, in parallel with the pre-blowing and final blowing of a container. That is to say, if compressed air flows into the preform, then compressed air also flows into the mold clamp. It is also known that the mold clamp is pressurized with compressed air independently of the pressurizing of the preform, retained in the blow mold, with pre-blowing air or final blowing air. To this end, provision is made, for example, for a separate valve which is electromagnetically actuated, for example. Via the actuating of this electrically operated valve, the duration of blowing pressure application to the mold clamp can be freely selected. This is preferably used in the case of blowing methods in which the duration and the force of the clamping has to be controlled independently of the blowing pressure.
If the mold clamp is pressurized with blowing pressure in parallel with the pre-blowing and final blowing, it is disadvantageous that the development of force which is generated via the blowing air in the mold clamp lags behind a rapidly developing container bubble in the blow mold. In this case, the container material can be squeezed into not yet fully closed mold gaps between the segments of the blow mold and a so-called parting line is created, which corresponds to a material surplus on the outer wall of the blown container in the region of the mold gaps. Containers with a parting line are usually treated as scrap or considered to be defective.
In the case of conventionally operated mold clamps, it has been shown to be disadvantageous that with an unfavorable ratio of the holding volume of the mold clamp in relation to the volume of the container which is to be produced frequently leads to the occurrence of parting lines. The introduction of blowing air into the preform with simultaneous introduction of the blowing air into the mold clamp occasionally leads to the preform quickly expanding and already butting against the walls of the blow mold before the mold clamp closes the mold gaps between the segments of the blow mold. It occurs for example because the feed line cross section for the introduction of the blowing air into the preform is considerably larger for constructional reasons than the feed line cross section of the mold clamp. The different feed line cross sections, with equal inlet pressure, lead to different volumetric flows and therefore to filling of the preform or of the mold clamp at a different rate. This especially makes itself felt in the case of blow molding of small containers in which the holding volume is small compared with the volume of the mold clamp.
A blowing station with a pneumatic mold clamp is known from DE 10 2011 110 962 A1, in which a supply pressure for the mold clamp is tapped from a pneumatic supply line of the blowing station.